Dissimilar metal joining method

Abstract

In the present invention, when spot welding a cold rolled steel sheet and an aluminum alloy sheet, pre-electrification is performed ahead of time in conditions wherein the sheet thickness (ta) of the aluminum alloy sheet and the sheet thickness (ts) of the cold rolled steel sheet that are to be joined are in specific ranges, dust is not generated between the steel sheet and an electrode, and the aluminum alloy sheet side is not caused to melt, and afterwards, main spot welding is performed. The welding conditions of the main spot welding and the pre-electrification further satisfy a relational expression that is determined by each electrification condition and each sheet thickness.

1 . A dissimilar metal joining method of a cold rolled steel sheet and an aluminum alloy sheet by spot welding, the method comprising:
 setting the thickness ts of the cold rolled steel sheet to be joined at 0.3 to 3.0 mm, and the thickness to of the aluminum alloy sheet to be joined at 0.3 to 4.0 mm, and further,   before performing spot welding to form a nugget under respective conditions of an inter-electrode electrode force: 2.5 to 4.5 kN, a welding current Iw: 14 to 26 kA, and a weld time Tw: 200 msec or less per a steel sheet thickness (ts) of 1 mm,   previously performing pre-welding under the conditions, under which expulsion is not caused between the steel sheet and electrodes, and the aluminum alloy sheet side is not molten, of an inter-electrode electrode force: 2.5 to 4.5 kN, a welding current Ib: 6 to 12 kA, and a weld time Tb: 200 msec or less per a steel sheet thickness (ts) of 1 mm with respect to the electrodes brought in contact with the mutual sheets, and   performing the spot welding in 1 second thereafter,   wherein the conditions for the spot welding and the pre-welding further satisfy the following relational expression determined by the respective welding conditions the respective thicknesses:
   Relational expression: 20≦( Ib   2   ×Tb+Iw   2   ×Tw )× ts/ta≦ 55(kA 2 ·sec).